In rotary pumps, fans, compressors, agitators, etc., the shaft projects through the casing in a zone known as the “stuffing box” or “packing box.” These terms derive from the fact that in order to separate two environments, namely, the environment within the pump, fan, compressor, agitator, or the like, and the atmosphere, and to prevent leakage from one environment into the other, some material had to be stuffed or packed around the shaft where it passed through its casing. For many years soft packing was the material that was most often employed for this service.
In operation, rotating shafts may be displaced both radially and axially. Small inaccuracies resulting from machining, manufacturing, and assembly produce radial displacement, and differential thermal expansion causes axial displacement. Therefore the resulting seal must be flexible. Another characteristic of the seal is compactness, which is dictated by design limitations of the equipment. A common type of rotating shaft seal consists of packing composed of fibers which are first woven, twisted, or braided into strands, and then formed into coils, spirals, or rings. To ensure initial lubrication and to facilitate installation, the basic materials are often impregnated with a binder and a lubricant. Historically, commonly employed materials were asbestos fabric, rubber and duck, flax, jute, and metallic braids. In the manufacture of packings, the proper grade, and type of lubricant is selected for the packing. It is usually desirable to replenish the lubricant during the normal life of the packing; otherwise the packing becomes hard, and loses its resiliency. As the packing ages, friction increases producing heat, which can shorten the life of conventional packing and increase operating costs. An effective auxiliary device frequently used with packing is a lantern ring, sometimes called a seal cage. The lantern ring provides an annulus around the shaft for the introduction of lubricant. The lantern ring is also used to introduce liquid for cooling, and in some cases heating, to prevent the ingress of atmospheric air, and to prevent the infiltration of abrasives from the process liquid. The chief advantage of packing over other types of seals is the ease with which packing can be adjusted or replaced. The major disadvantages of a packing type seal are that it historically has a short life, adjustment must be frequent, and there is a need for some leakage to provide lubrication and cooling.
Mechanical seals have largely replaced packing seals on rotating shafts. Mechanical seals are reliable, have long life, and, in general, operate with no visible leakage. The term “mechanical seal” designates a prefabricated or packaged assembly that forms a running seal between the flat precision finished surfaces. The prior art teaches that all mechanical seals contain four common elements: a rotating seal ring, a stationary seal ring, a spring-loading section for maintaining seal-face contact, and static seals.
With the advent of centrifugal pumps, the pump shafts had been made solid and the type packing used at the time was made of an oil lubricated asbestos, which, after extended use, became very abrasive, causing wear damage to the shaft. The cost of replacing the entire solid shaft was very expensive and shaft sleeves were then introduced to reduce cost. The inexpensive shaft sleeve replacement cost only a fraction of the solid shaft replacement.
However, centrifugal pump shafts were made to operate at much higher speeds than the former rotary pump shafts which led to much greater shaft wear. Cost savings failed to improve because of the necessary frequency of replacement. Also the sleeve had a slight dead air space between it and the shaft interfering with or limiting heat transfer to the shaft.
Next, a water flush system was introduced to cool and lubricate the stuffing box surrounding the shaft. This introduced an additional new problem: the new sleeves were typically manufactured from stainless steel to provide service in corrosive environments. It should be noted that the stainless steel sleeve is protected by a naturally occurring phenomenon in which the stainless steel is passivated by the presence of chromium in the alloy. However, free oxygen present in flush water unites with the chromium in the stainless steel to form chromium oxide (Cr2O3) which is second only to diamond in hardness (Moh's Scale). Chromium oxide is used as a buffing medium to polish metals and is fused to form hard ceramic coatings. Chromium oxide is formed onto the shaft sleeve as the chromium migrates to the surface of the sleeve, resulting in the hard oxide coating which is wiped off and deposited onto the packing surface which polishes the sleeve surface destructively. This explains why, in simple water systems, the sleeve can be observed to take on a polished appearance and be badly worn when no abrasives are visibly present in the media. The worn portion of the sleeve then allows leakage into the environment causing housekeeping and pollution problems.
In the present invention, water is not allowed between the packing and the shaft to cause excessive wear. Water is simply introduced into the fluid media being pumped without the packing being turned into abrasive rings which would cause excessive wear.
What is needed is a self-lubricating bushing that also can serve as a lantern ring to control coolant flow. Furthermore, what is desired is a sealing apparatus that acts comparable to mechanical seals, which require much less maintenance than packing and generally can operate for longer periods between adjustments or replacement. Further, what is needed is a bushing that can accommodate minor wear and imperfections in a shaft. Highly desirous is a bushing or lantern ring that can be installed with a minimum amount of disassembly, and that can accommodate relatively high temperature and high revolutions.